This invention relates generally to electronically controlled brake systems for rail vehicles, and more particularly, to an electronic grade speed control and method therefore for a rail vehicle for automatically adjusting a train brake application as required to maintain a preselected train speed.
From the inception of the early Westinghouse Air Brake, until the present time, compressed air has been the medium by which brake control signals have been transmitted through a train of railroad freight cars, as well as the force by which friction retardation is applied through brake shoes that engage the car wheel treads during braking. With the advent of electro-pneumatic (ECP) brake control systems, the capability of the air brake has been extended beyond that which could be achieved with conventional pneumatic brake control systems. The improved capabilities are due primarily to the fact that the brake control signal can be transmitted instantaneously to each car in the train, whereas propagation of a pneumatic control signal is limited to a value approaching the speed of sound.
In a freight train, a number of articulated rail cars are typically interconnected by a brake pipe which supplies pressurized fluid from a main reservoir on a locomotive. Each car normally has on-board a brake pipe, a reservoir which is charged with pressurized fluid from the main reservoir, an exhaust device and a fluid pressure activated brake cylinder device. In some cars, a pneumatic control valve may also be present in conjunction with an electronic controller of an ECP freight brake control system.
In an ECP system, the electronic controller operates solenoid actuated valves which control the access of pressurized fluid between the reservoir, the brake cylinder and the exhaust device.
The pressure in the brake pipe can be controlled from the locomotive by the train engineer. Conventionally, there are three different types of brakes controlled by the engineer on the locomotive. The first is an xe2x80x9cindependent brakexe2x80x9d which are the brakes on the locomotive only. The second type is referred to as a xe2x80x9cdynamic brakexe2x80x9d and pertains to the use of the locomotive engines to provide a retardation force for the train. The third type is the xe2x80x9ctrain brakesxe2x80x9d or xe2x80x9cfriction brakes,xe2x80x9d which refers to the pneumatic brakes on each of the rail cars. With respect to the friction brakes, a reduction in the brake pipe pressure by the train engineer signals either the pneumatic control valve or the electronic controller to apply the brakes on the rail car. The level of braking force to be applied is generally a function of the amount of reduction in brake pipe pressure. Although the electronic controller can utilize pressure sensors to detect changes in the brake pipe pressure, the train engineer could also electrically transmit a command signal to the electronic controller on each rail car instructing it to apply a selected amount of braking force. Similarly, an increase in the brake pipe pressure is a signal to release the brakes on the rail car. Also, as with applying the brakes, a command signal can also be transmitted to instruct the electronic controller to release the brakes.
While pneumatic braking is used for a number of purposes in normal train operation such as to slow or stop a train or to control inter-car dynamics (slack, run-in, run-out) special consideration can be given to the operating condition when braking is used to maintain the speed of a train on a descending grade. During this condition, the friction brakes are often used to supplement the dynamic braking supplied by the locomotive in the train. When grade braking, only up to about one-half of the available full service brake cylinder pressure is typically used, as needed, to assist in balancing the gravitational grade acceleration force imparted on the train. If the total train retarding force exactly matches the grade accelerating force, acceleration is zero and velocity is held constant. If the total retarding force is greater, velocity decreases.
Freight trains can often be dozens, even hundreds, of cars long, resulting in an extremely large moving mass, which requires an equally large degree of braking force to control. Consequently, it can be difficult to maintain a constant pre-selected train speed when the train is traveling on a descending grade. Control of the speed of the train on a descending grade can be problematic, especially if the speed of the train increases beyond a safe degree or if the reservoirs become overly depleted such that control over the speed of the train is compromised.
Historically, a problem has sometimes arisen because the braking control systems typically had no provision for a graduated release of brake cylinder pressure. Only after the brake cylinder pressure had been exhausted could a new brake application be applied at a different level. Thus, to alter the level of braking after an application was initiated, the brake cylinder had to vented. Moreover, venting the brake cylinders depletes pressure in the system much more quickly than it can be replenished from the main reservoir. Consequently, applying and exhausting the brakes successive times can quickly deplete pressure reservoirs below a level capable of controlling the speed of the train. Clearly, this can be particularly undesirable on a descending grade. Thus, maintaining the speed of the train within a tight range on a descending grade is highly desirable. However, using a graduated release brake valve, it is possible to selectively increase or decrease the brake cylinder pressure any number of times without entirely exhausting the brake cylinder pressure. A graduated release brake valve is disclosed in co-pending U.S. patent application Ser. No. 09/894,053, which is hereby incorporated herein by reference.
A grade speed control system for a railway freight vehicle according to the invention can be accomplished by a speed control system having a microprocessor which receives input from various sources, such as from both the dynamic brake and the independent brake on the locomotive, as well as the brake cylinder on each rail car. Additionally, values can be input to the microprocessor for the desired train speed, the actual train speed, and the constants and equations utilized to convert the raw data into the values necessary to derive the brake cylinder pressure adjustment needed to implement the speed control functions of the grade speed control system. Additionally, the grade speed control system can communicate with a brake cylinder control device, such as, for example, an electronic controller, on each rail car in order to increase or decrease the brake cylinder pressures to control the train speed.
In implementing the grade speed control, when the train begins down a descending grade, the operator will typically set the desired level of dynamic brake and then gradually apply the friction brakes as required to generally balance the gravitational accelerating force of the grade and maintain the desired train speed. The grade speed control may then be activated by setting a switch and inputting the desired train speed to the brake control microprocessor on the locomotive. The microprocessor can monitor actual train speed, calculate the acceleration, and compare actual speed to target speed. If the actual speed differs from the target speed by more than a predetermined amount, a target acceleration is calculated. From the target acceleration, a brake cylinder pressure adjustment can be derived to achieve the target acceleration and bring the actual speed of the train to the target speed in a reasonably brief period of time. This sequence can be reiterated as the train progresses down the grade, automatically adjusting train braking effort as required to maintain the target speed within a tight range.
A new target speed may be designated at any time, and the grade speed control system can automatically adjust train speed to match the new target speed. Similarly, the dynamic brake setting may be changed, and the brake control microprocessor can automatically compensate with an appropriate adjustment to the brake cylinder pressures. Additionally, if either train speed or brake cylinder pressure approaches an excessive level, the operator can be promptly warned by the system.
The grade speed control system can also vary the level of brake cylinder pressure adjustment based on the magnitude of the difference between the target speed and the actual speed. For example, where the difference between the actual speed and target speed is greater than a predetermined amount, a higher value can be utilized in the equations from which brake cylinder pressure adjustment is derived in order to implement a critical speed control adjustment. Similarly, if the magnitude of the difference between the actual speed and the target speed is sufficiently small, a lower value can be used in the brakes cylinder pressure adjustment equations to implement a normal speed control adjustment. Moreover, to avoid overshooting the target speed, the brake cylinder pressure adjustments can be modified as the actual speed approaches the target speed. The prevailing acceleration, as controlled by the existing brake cylinder pressure, can preferably be derived from successive velocity measurements. The change in brake cylinder pressure can then be predicated on the difference between the prevailing train acceleration and the target acceleration. By reiteratively carrying out this process, the brake cylinder pressure can be automatically and continuously controlled to maintain the speed of the train within a tight range.
Other details, objects, and advantages of the invention will become apparent from the following detailed description and the accompanying drawings figures of certain embodiments thereof.